A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a navigation function of the vehicle guides a driver to a destination through a calculated route. Such a navigation system detects the position of the vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium such as a DVD (digital versatile disc). Typically, the navigation system displays a map image on a screen while superimposing thereon a mark representing the current vehicle position.
When a start point and a destination are specified, the navigation system operates such algorithms as an A* search algorithm and its variants to calculate a route between the start point and the destination. In establishing a route to the destination by such an algorithm, in some cases, the navigation system may produce a calculated route that involves a freeway-ramp-freeway situation. Here, a freeway-ramp-freeway situation refers to the condition where the route is initially on a freeway, then exits the freeway, and immediately re-enters the same freeway.
An example of such a freeway-ramp-freeway situation is shown in the schematic diagram of FIG. 1 which shows a case where a freeway is excessively curved. In this situation, the navigation system may establish a calculated route that exits at an exit ramp 11 and enters the same freeway 9 at an entry ramp 13. This is because, taking off the freeway 9 to reenter the same freeway after the curve can be effective in reducing an overall cost, i.e., a driving distance and a time, since the freeway 9 is curved significantly.
Although the overall distance may become shorter, there are other considerations that may not favor the use of the freeway-ramp-freeway situation shown in FIG. 1 on the calculated route. One consideration is a driving speed that may be reduced by exiting the freeway to take a normal (surface) street, which may destroy the purpose of using the freeway-ramp-freeway situation. Another consideration is maneuverability that is affected by the required maneuver to exit and reenter the freeway.
More importantly, driving a vehicle via the freeway-ramp-freeway situation may violate local traffic regulations. Therefore, in some cases, such a calculated route that includes the freeway-ramp-freeway situation described above may not be desirable. Thus, there is a need of a new method and apparatus for calculating a route and detecting a freeway-ramp-freeway situation in the calculated route and correcting the freeway-ramp-freeway situation, thereby establishing an optimum guidance route to the destination in view of the considerations noted above.